Conventionally, it has been common to allow ionizing radiation, such as X-rays or gamma rays, to transmit through an object being a specimen such as food, drugs, luggage, human or animal body, etc. and make an inspection based on a transmission ionizing radiation image to determine the object, for example an existence of a different kind of substances in an object. For such an inspection, an ionizing radiation image acquiring device, including an ionizing radiation source for irradiating ionizing radiation to the object, and a linear line sensor for detecting a transmission image of ionizing radiation irradiated to the object from the ionizing radiation source, has been used.
Use of a dual energy is known in this field. In dual energy imaging, two separate image frames are taken. Each image frame is at a different energy level or a radiation spectrum. This may be achieved by using different filtering or different acceleration voltage, etc. The dual energy is based on the fact that dominant interactions of ionizing radiation in materials differ at different energies. This difference is material specific. This makes it possible to enhance a detection of certain unknown materials. Especially with respect to known thickness of known materials, by removing the known material form the image data. For the purpose of these calculations, high energy data and low energy data are taken from the same volume of the material.
Dual energy imaging can basically be configured in three ways. 1) By taking two separate images one after another at different energies, either using two different tubes, or one tube with different energy and filtering, and also matching detectors. 2) By imaging the object in one shot with one tube setting, but using a sandwich detector, where a top layer is optimized for low energy, and filtering the beam for the second detector optimized for the high energy. 3) By placing the detectors not on top of each other but one after another. Consequently, the detectors can be placed in parallel to each other. The dual energy system can be based on the side by side, i.e. in parallel, detectors.
Publications U.S. Pat. No. 5,841,832A and US2010119038A1 discusses information that can be regarded as useful for understanding the background.